Regulators launched a safety alert last night after warning that thousands of packets of the over-the-counter painkiller Nurofen Plus could have been filled with an anti-psychotic drug.

The Medicines and Healthcare products Regulatory Agency (MHRA) issued its alert after reports that some batches of Nurofen Plus contained individual blister packs of another drug, Seroquel XL 50mg. Strips of Seroquel were initially found in three packets of Nurofen Plus in branches of Boots in the Victoria, Beckenham and Bromley areas of London.

However since thousands of the 32-tablet packs could potentially be affected, the MHRA urged "extra vigilance".

Batches of the packs could have ended up in pharmacies across the UK. Each batch contains between 4,000 and around 7,500 packs, amounting to about half a million tablets.

The two drugs are very different. Nurofen Plus, made by Reckitt Benckiser, is an over-the-counter tablet containing codeine and is one of the most popular brands in the £500m painkiller industry. Seroquel XL, manufactured by AstraZeneca, is a prescription-only anti-psychotic drug used to treat several disorders including schizophrenia, mania and bipolar depression.

People who accidentally take Seroquel may experience sleepiness and are advised not to drive or operate any tools or machinery until they know how the tablets have affected them. Side effects of the drug include dizziness, headache and sleepiness.

The large capsules of Seroquel XL 50mg have gold and black packaging while the Nurofen Plus tablets are smaller and have silver and black packaging.

The MHRA stopped short of recalling Nurofen Plus, opting instead for pharmacists to be extra vigilant in dispensing the painkiller.

Ian Holloway, from the MHRA's defective medicines report centre (DMRC), said: "People should check to see if they have any affected packets of Nurofen Plus.

"If you do, return them to the pharmacy where you bought them from. He said that anyone concerned could ring the MHRA's report centre on 020 3080 6574. "If you have taken a tablet and you have any questions, speak to your GP."

Reckitt Benckiser issued a statement saying there had been three cases identified so far, all confined to south London.

It said "serious investigations" were under way to establish how the mix-up occurred, especially as Seroquel XL is manufactured by another drug firm, AstraZeneca.

"After careful review of the manufacturing system, manufacturing errors by the makers of Nurofen Plus or Seroquel XL are not thought to be part of the cause at this stage," the statement said.

A number of theories were offered as explanation. The first is that this was a simple mix-up is believed to have happened at a wholesalers.

However, given that the two different manfacturers are involved and the tablets are made and packaged in separate sites in different parts of the country, neither company is ruling out sabotage. Siân Boisseau, director of Virgo Health, a public relations firm which represents Reckitt Benckiser, told the Guardian that there has been a "suggestion that the packets were deliberately put in the wrong boxes.

"Manufacturing and supply errors by the makers of Nurofen Plus or Seroquel XL are not considered to be part of the cause at this stage."

A statement from AstraZeneca said: "Patient safety is the primary concern of AstraZeneca and the company is taking this issue seriously. AstraZeneca is collaborating with the MHRA and Reckitt Benckiser to investigate the root cause.

"The MHRA has issued an alert to all pharmacists and healthcare trusts to make them aware. If someone believes they have taken Seroquel XL by mistake they should contact their physician, and return all packaging to the pharmacist."

Dr Aomesh Bhatt, medical director for Nurofen Plus, pointed out that the painkiller cannot be purchased from the shop floor. "Nurofen Plus is a pharmacy-only medicine which means it is behind the pharmacy counter. We are in the process of working to ensure the Nurofen Plus packs are double-checked by pharmacy staff before they are handed to customers."